Our robot is basically U shaped; we cut a piece out of the front of the AndyMark frame and so the front is now two roughly 4.5 inch caps in front of the wheels, very similar to the open frame kit AndyMark sells.

The rules said in one place that sides shorter than 6 inches are legal so long as the entire side is covered in bumper, but a different local team said that might not be the case and that we'd have to extend the caps to 6 inches long. May I ask what the official ruling on this is?

Our robot is basically U shaped; we cut a piece out of the front of the AndyMark frame and so the front is now two roughly 4.5 inch caps in front of the wheels, very similar to the open frame kit AndyMark sells.

The rules said in one place that sides shorter than 6 inches are legal so long as the entire side is covered in bumper, but a different local team said that might not be the case and that we'd have to extend the caps to 6 inches long. May I ask what the official ruling on this is?

Thank you for the help.

From 2018 FRC Inspection Checklist Rev 1.1

Bumpers must provide protection of at least 6” on both sides of all outside corners. (Wood within ¼” of corner) <R23>

I know that the official site is sometimes not as clear as we'd like. That's because they don't review specific designs.

I have not seen your specific design, but I can guess from your question that it has two short frame segments on one end, with an opening between them so that one of your robot's mechanisms can get access to a game piece.

Those short segments are NOT considered "sides" of the FRAME PERIMETER, which is defined in the Glossary section of the Game Manual. The FRAME PERIMETER is not the same thing as the frame. It is a convex polygon defined by the robot's outermost vertices.

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Our robot is basically U shaped; we cut a piece out of the front of the AndyMark frame and so the front is now two roughly 4.5 inch caps in front of the wheels, very similar to the open frame kit AndyMark sells.

The rules said in one place that sides shorter than 6 inches are legal so long as the entire side is covered in bumper, but a different local team said that might not be the case and that we'd have to extend the caps to 6 inches long. May I ask what the official ruling on this is?

To translate it, those frame caps are not the entirety your FRAME PERIMETER. They're part of it. Your particular* FRAME PERIMETER is actually a rectangle that goes around your robot. So you need 6" of bumper covering each of those corners. And that needs to be supported at both ends, so those frame caps are going to need to be 6" long.

*Not all FRAME PERIMETERS are rectangles, though most are.

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The entire side, including the cut out part, is considered the FRAME PERIMETER. If you were take a giant rubber band and wrap it around the frame of your robot, that is considered your FRAME PERIMETER. So the gap, and the two small metal pieces extending to the corners, are considered as a single side and you must have 6" from each corner covered by the bumper.

Unfortunately, to be in compliance with R32 you need to extend the front frame sections so that the wood of your bumpers are backed by your metal frame.

Also, last year (I was told by a mentor that went with the kids to get the robot inspected) when we had the same configuration, that our bumpers were 1/4" short. The inspector said he would let us compete at that event, but we would have to make new ones if we progressed to the next event. (We didn't) But the lesson we got was to make sure they're just a smidge larger than the minimum.

Looks like you have all the answers you need. I just wanted to say Kudos for asking. So many will leave bumpers till the end or even a post bag and tag activity. In case no one has said it in so many words, one of the significant things about the bumper rules is they define your frame requirements within R32, the bumper support requirements. They're written this way to give maximum flexibility in your design but it trips up a lot of teams.

"The rules said in one place that sides shorter than 6 inches are legal so long as the entire side is covered in bumper..." Only true if the side is less than 6" vertex to vertex as in Figure 8-3, as others have said.
Look at Fig 8-2 in the lower right corner of the top diagram. It specifically addresses your situation.